Star paths reveal secrets of Milky Way's black hole

The central part of the Milky Way is shown in this near-infrared image taken by the NACO instrument on the Very Large Telescope

(Image: ESO/S Gillessen et al.)

Video: Sixteen years of observations of stars around the galactic centre are assembled in this video. The real motion of the stars has been accelerated by a factor 32 million. The individual images have been shifted and stretched to the same scale and orientation (Courtesy of ESO/R Genzel and S Gillessen)

Video: One star, called S2, was observed over its complete 15.8-year-long orbit around the Milky Way’s central black hole. The star approached the black hole to within one light day, which is only about five times the distance between the planet Neptune and the Sun. (Courtesy of ESO)

Astronomers have crunched 16 years of data to make the most detailed observations yet of the stars orbiting the centre of our galaxy, bolstering the case that a monstrous black hole lurks there.

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Black holes are invisible but can be detected by their influence on nearby stars. By observing the motion of 28 stars orbiting the Milky Way’s central region, Reinhard Genzel of the Max-Planck-Institute for Extraterrestrial Physics in Germany, says his team has delivered “the best empirical evidence that supermassive black holes do really exist”.

Genzel’s team found that the black hole, known as Sagittarius A*, is about 4 million times as heavy as our Sun, in line with previous estimates. They also used the observations to work out that the Earth is 27,000 light years away from the centre of the Milky Way, also agreeing with previous estimates.

One star, S2, was orbiting so fast that it completed an entire revolution of the black hole over the 16-year period. Observing one complete orbit of S2 was critical to the high accuracy reached and to understanding the region, says the team.

The research could also help astronomers understand the formation of the Milky Way. “The centre of the galaxy is a unique laboratory where we can study the fundamental processes of strong gravity, stellar dynamics and star formation that are of great relevance to all other galactic nuclei, with a level of detail that will never be possible beyond our galaxy,” explains Genzel.

High accuracy

The observations are the culmination of work that began in 1992 at the European Southern Observatory’s (ESO) 3.5-metre New Technology Telescope. It was then pursued at ESO’s Very Large Telescope (VLT), an array of four 8-metre telescopes. Both telescopes are based in Chile.

Future work will require a higher resolution than the telescopes used for this study can currently offer.

A project called GRAVITY will combine light from the four telescopes at the VLT.

According to Frank Eisenhauer, principal investigator of the GRAVITY project, this should improve the accuracy of the observations by a factor of 100 and “has the potential to directly test Einstein’s general relativity in the presently unexplored region close to a black hole”.